The invention relates to a circuit arrangement and a method for compensating for temperature with a sensor operating by the eddy current principle and preferably comprising a measuring coil for measuring physical conditions, and with an evaluation unit for evaluating the measuring signal of the sensor, with the sensor and the evaluation unit being interconnected via a connection line.
Circuit arrangements and methods for compensating for temperature have been known in practical operation for a long time. In the case of circuit arrangements which operate by the eddy current principle, the impedance change of the measuring coil of a sensor is measured, which results from the approximation of an electrically conductive object of measurement. Subsequently, the impedance change is evaluated, so that a measuring signal results, which reflects as much as possible in a linear relationship, for example, the spacing between the sensor and the object of measurement. However, since both the impedance of the measuring coil used for the measurement and the electric and magnetic conductivity of the object of measurement and, thus, the reaction of the object of measurement to the sensor also change with the temperature, a corresponding temperature compensation is required for obtaining a correct measuring signal. At the same time, it is to be considered that the temperature influence requires different compensation values as a function of the measuring distance.
A possibility of accomplishing this, lies, for example, in measuring the temperature of the sensor. It is then possible to use the temperature measuring signal for selecting corresponding correction values from a stored table, or for interpolating them, if need be, and for correcting therewith the measuring signal. As an alternative, it is possible to offset the temperature measuring signal directly against the measuring signal, for example, by a simple or multiple addition and/or by multiplying correction factors with the measuring signals. This may occur before and/or after a linearization of the measuring signal, possibly even by an additional nonlinear change of the temperature measuring signal before the correction step. In this manner, it is possible to attain a simple temperature compensation.
However, it is often necessary to arrange the sensor in a location removed from the evaluation unit, for example, for reasons of space or for reasons of environmental conditions. Frequently, there is likewise no space directly available on the sensor to perform an electronic preprocessing of the measuring signal. It is therefore necessary to connect the sensor via a connection line to the evaluation unit. However, this connection line represents an additional impedance, which itself is again temperature-dependent and, thus, also generates temperature-dependent measuring errors. In many cases, the temperature of the sensor is also not identical with the temperature of the connection line, so that a temperature compensation, which is derived only from the temperature of the sensor, does not simultaneously compensate the measuring error caused by the connection line.
EP 0 049 304 B1 discloses a circuit arrangement and a method for compensating for temperature with a sensor operating by the eddy current principle for measuring physical conditions, and with an evaluation unit for evaluating the measuring signal of the sensor. The sensor, which includes a measuring coil, and the evaluation unit are interconnected by a connection cable. A suitable adjustment of the impedances of the measuring coil, the object of measurement, and the connection cable results in a temperature response, which adequately compensates with the method disclosed therein also the influence of the connection cable. However, this adjustment and conformity of the impedances is to be realized only in special cases. Since the sensor is normally used toward different objects of measurement with correspondingly different temperature-dependent impedance changes, and since, as a result of the use, often narrow limits are set to the possible dimensions of both the measuring coil and the used wire diameter, major measuring errors also occur in the case of both the circuit arrangement and the method disclosed in EP 0 049 304 B1 because of the different temperatures of the sensor and the connection cable.
It is therefore an object of the present invention to provide both a circuit arrangement and a method for compensating for temperature of the initially described type, wherein temperature caused interferences, in particular temperature caused interferences produced by the connection line, are minimized or avoided to the greatest extent with a constructionally simple layout.